Device for detecting the properties of a web of material transported in the longitudinal direction

ABSTRACT

A device for detecting properties of a web of material has a crossbar extending across the web of material. An infrared spectrometer having a holographic grating is provided an has an input side and an output side. Infrared detectors are arranged at the output side of the infrared spectrometer and are formed by a detector matrix having n lines and m rows of infrared sensitive individual sensors. A plurality of optical waveguides are provided, each waveguide having an entrance area and an exit area. The entrance area is fastened to said crossbar, located in vicinity of the surface of the web of material and is oriented towards said surface. The exit areas of the optical waveguides are connected to the input side of the infrared spectrometer. The optical waveguides are arranged side by side in one line at this input side, so that infrared spectra inputted into the entrance areas of the individual optical waveguides appear in rows side by side at the output side of the spectrometer, and the spectra of up to m optical waveguides are distributed and detected in up to n spectral areas.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a device for detecting properties of a web ofmaterial conveyed in longitudinal direction, e.g., a web of paper, a)with a plurality of optical waveguides having their entrance areaslocated each in the vicinity of the surface of the web of material andoriented to said surface and being fastened to a crossbar extendingacross the web of material, b) with an infrared spectrometer to whichinput the exit areas of the optical waveguides are connected and c) withinfrared detectors at the output of the infrared spectrometer.

2. Description of the Prior Art

Devices and methods of this type are used in online process controllingin the continuous fabrication of sheet materials such as paper, textile,foils, and so on. The device permits to specifically chemically detectmost of the processing chemicals. In this way, even the fastestfabrication procedures as they occur, e.g., in paper making andfinishing may be monitored with sufficient accuracy. The device istherefor suited for all continuous manufacturing processes in which oneor more components determine the quality. For this reason, it is mountedon mixers, calenders, padding machines, doctor blades, steamers anddriers in order to be able to more constantly regulate components,additives, applications of products in coating, impregnating, laminatingas well as dampness in the course of drying.

The device of the type mentioned above has been previously proposed inDE 197 09 963. In this device, the individual optical waveguides, whichare also known as optical fibers and are arranged on the crossbar, areled to a switch that consecutively brings each and every single opticalfiber in optical contact with a transfer fibre connected to aspectrometer for a period of time. This procedure is also known asmultiplexing. Accordingly, all of the individual fibers are connectedone after the other to the spectrometer for a short time period overwhich the optical signal detected by the corresponding individualoptical fiber is interpreted.

This embodiment presents the disadvantage that on one side amechanically operating device, viz., the switch, is used that has toprovide optical contact between different optical fibers with very highaccuracy. Practical use compounds the difficulty of durably operating aswitch in such a manner that it accurately connects and transmits theoptical signals. On the other side, the switch provides little time onlyto detect the signal of one single optical fiber. This signifies thatthe optical signal delivered by this fiber is only detected for a verysmall share of the overall time in time average. This makes it difficultto completely detect a web of material. It is not possible to thusobtain an overall picture of the web of material as it is increasinglydemanded.

It is the object of the present invention to develop the device of thetype mentioned above in such a manner that a mechanical switch, which isalways complicated in manufacturing and in practical operation, isrelinquished and that a higher detection rate is achieved.

SUMMARY OF THE INVENTION

Starting from the device of the type mentioned above, the solution tothis object is to provide the infrared spectrometer with a holographicgrating, to have the optical waveguides arranged side by side in oneline at the input of the spectrometer in such a manner that the infraredspectra of the signals of the individual optical waveguides appear inrows side by side at the output of the spectrometer and that theinfrared detectors are formed at the output by a detector matrix with nlines and m rows of infrared sensitive individual sensors, the spectraof up to m optical waveguides may be distributed and detected in up to nspectral areas.

This device makes it possible to concurrently monitor m areas of the webof material. The m entrance areas of the optical waveguides are orientedonto these m areas. The number of the entrance areas is preferably lessthan m, e.g., 0.8 times m, 0.5 times m. Of each and every single area ofthe web of material observed through the input range, a complete IRspectrum is permanently imaged on the matrix of the detector. It isdiscretional how this matrix is interpreted, but it is in any casepossible to electrically detect and interpret permanently the spectraamounting to a total of up to m spectra.

Accordingly, the device according to the invention provides thepossibility to virtually completely sample the web of material to betested and examined. In other words, it becomes possible to obtain anoverall picture of this web of material. As compared to the devices ofthe art, the detection rate is substantially higher in any event.

The detector matrices used are commercially available arrays as they areparticularly utilized in cameras. Detector matrices as they may be usedin the invention are offered by the firm Rockwell or by SensorsUnlimited, Inc. for example and are known as focal plane arrays. Theyare typically designed for the wavelength range of 0.9 to 1.7micrometers. The number m of rows is adapted to the infraredspectrometer, the individual spectra are to substantially illuminate theindividual rows. The number of rows of the detector matrix is usuallyhigher than the number of optical waveguides, in a preferreddevelopment, at least one line is always left unused between two spectrain order to achieve a clear separation between neighboring spectra. Itis moreover absolutely possible and even intentional to combine and toelectrically operate in common several neighboring individual sensorsand accordingly pixels. The device according to the invention utilizesdetector matrices as they are in fact to be found on the market. Withthese matrices, the size of the individual sensors is the smallestpossible so that high resolution is achieved. This however is notnecessary for the device according to the invention.

Owing to the use of a holographic grating, it is possible to preciselydevise the grating, the design being accurately calculated for theselected position of input and output of the spectrometer, respectively.In the preferred embodiment, the holographic grating has been given acylindrical convex shape. In another preferred embodiment, the image ofthe input is created on the output by way of mirror optics. Therefore, aconcave mirror is preferably arranged between input and grating, anotherconcave mirror being preferably provided between the grating and theoutput.

Ratios of between 0.5 to 1 and 1 to 0.5 proved to be appropriate as animage ratio between input and output of the spectrometer, the imageratio of preference being approximately 1 to 1. Owing to the relativelysmall areas of the commercially available detector matrices thissignifies very small surfaces for the input of the spectrometer whichimplies the need to use absolutely thin optical fibers at the input.Typically, optical fibers 50 to 60 micrometers in diameter are used.They are packed tightly into the input of the spectrometer, beingpreferably arranged in line. A zigzag line is also possible.

The use of relatively thin optical waveguides in the range of 50 to 60micrometers at the input of the spectrometer requires that the sameoptical waveguides be continued until they reach the entrance area orthat thicker optical waveguides, e.g., about 0.5 mm in diameter, be usedat the entrance area and to then couple these to the thinner opticalwaveguides. The way that has been described first presents the advantagethat it is not necessary to provide a coupling place between a thick anda thin optical waveguide but has the disadvantage that the extremelythin optical waveguide is very difficult to manipulate. This drawbackmay be addressed in that the optical waveguides are provided with arelatively thick cladding which simplifies their manipulation. Thiscladding only is dropped immediately in front of the input of thespectrometer where the optical waveguides are arranged in tight packingside by side. The disadvantage of the second way is that a number of upto n optical waveguides with a larger diameter has to be coupled to asame number of optical fibers with the small diameter. Although this istechnically possible, it is complicated. The second way presents theadvantage that it is better and easier to work with the relativelythicker optical waveguides. Both ways have comparable lightefficiencies. Although the thicker optical waveguides, which are used inthe second way, capture and transfer more light in the first place, thisachievement is got lost at the transition from the thick to the thinoptical waveguides so that in both ways each optical waveguide deliversapproximately the same light flux to the spectrometer.

The lighting fixture used for examining the web of material consists asfar as possible in point sources of light. Transmission or reflectionmay be utilized.

The interpretation of the signals of the detector matrix is carried outaccording to state of the art methods. Reference is made in thisconnection to WO 97/20429 for example. It relates to a control devicefor a CCD element.

The sensitivity of the individual sensors of the detector matrix varies.Methods of adjusting the sensitivity and also all the spectralproperties of the individual sensors are well known and are employed forthe device according to the invention. To compensate different darktensions, a chopper may be arranged in the light path of all of theindividual channels, it is preferably provided in immediate proximity tothe input of the infrared spectrometer. Moreover, the detector matrixmay be allocated temperature sensors so that the temperature there maybe detected. Through temperature, the parameters of the detector matrixmay then be compensated in as far as they depend upon the temperature.For this purpose, well-known methods such as FIR or PDS for example areemployed. In this connection, reference is made to the two followingpublications “Standardization of Near-Infrared SpectrometricInstruments, E. Bouveresse et al. Analytical Chemistry, Vol. 68, No. 6,Mar. 15, 1996” and “Transfer of Near-Infrared Multivariate Calibrationswithout Standards, Thomas B. Blank et al., Analytical Chemistry, Vo. 68,No. 17, Sep. 1, 1996.”

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and characteristics of the invention will becomeapparent in the other claims and in the following description ofexemplary embodiments that are not limiting the scope of the inventionand are explained in more detail with reference to the drawing.

FIG. 1: shows a perspective view of a web of paper conveyed over a roll,said web of paper being allocated a crossbar with optical fibersattached to it,

FIG. 2: a basic representation of an infrared spectrometer with anelectronic plotting unit,

FIG. 3: a top view on a detector matrix,

FIG. 4: a front view of part of an exit area of a plurality of opticalwaveguides and

FIG. 5: a partially sectioned side view of a connection region of twooptical waveguides having different diameters.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to FIG. 1, a web of paper 20 is guided over a roll 22, themachine direction is indicated by arrow 24. A crossbar 26 is fixedlyarranged above the roll 22, it has the shape of a beam arranged parallelto the axis of the roll 22. A plurality of optical fibers or opticalwaveguides 28 is fastened to said crossbar. They each have an entrancearea 30 that points downward in FIG. 1, it is oriented to the uppersurface of the web of paper 20 and is spaced from said surface by ashort distance of 2 mm for example. The various entrance areas 30 arearranged on a line parallel to the axis of the roll 22, they arefastened at regular intervals on the crossbar 26. They are substantiallyoriented normal to the web of paper 20.

The various optical waveguides 28 are individually guided toward ajacket 32 in which they are combined. In a first embodiment, the opticalwaveguides 28 have a diameter of approximately 50 to 60 micrometers. Ina second embodiment, the optical waveguides have a considerably largerdiameter of, e.g., 500 micrometers. They are reduced to opticalwaveguides of the first mentioned diameter of approximately 50 to 60micrometers, a device according to FIG. 5 is used for this purpose.

As shown in FIG. 2, the individual optical waveguides 28 exit again thecombining jacket 32 one by one and are combined to a line of directlyadjacent optical waveguides. FIG. 4 shows a top view of such a line. Thearrangement in line of the exit areas 34 of the optical waveguides 28,as it can be visualized as depicted in FIG. 4, is positioned at an inputside 36 of a spectrometer. The entrance slot of this spectrometer isvirtually realized in this way. The beam path of the light in thespectrometer can be visualized in FIG. 2. The light of the individualoptical waveguides 28 exiting the exit areas 34 first falls onto a firstconcave mirror 38, from there onto a holographic grating 40 formed on aconvex cylindrical surface, finally reaches a second concave mirror 42and is from there imaged on the output side 44 of the spectrometer. Theoverall ratio of the image realized by the optics described herein aboveis 1:1.

A detector matrix 46 is arranged at the output 44. The spectrometer isrepresented in such a manner that the exit areas 34 are lying above eachother across the plane of the paper, that is, one looks onto the lineararrangement of the exit areas 34 in longitudinal direction. A spectrumin the wavelength range of 0.9 to 1.7 micrometers is imaged on thedetector matrix. The other spectra of the individual light signals ofeach exit area 34 lie across the plane of the paper below the spectrumrepresented.

FIG. 3 shows a view onto the detector matrix 46. For greater ease it hasrelatively few individual sensors 48, namely a total of ninety-nineindividual sensors that are divided into n=nine lines and m=elevenlines. Actually used detector matrices 46 have many times this number ofindividual sensors, e.g., 128×128 individual sensors.

In the left portion of the detector matrix 46, three spectra arerepresented side by side for the sake of explanation. The first spectrumsubstantially illuminates the first row m=1, but in parts also the rowm=2. The second spectrum substantially illuminates the third row m=3,but in parts also the two neighboring rows. The same is true for thethird spectrum which substantially illuminates the fifth row m=5, butalso illuminates the neighboring rows in the process. Only theindividual sensors of the first, third and fifth etc. row are utilizedfor interpretation. It is possible to increase the distance between therows used. As a result, the spectra of neighboring optical fibers can bereadily separated on one side and on the other, specific adjustment ofthe optics etc. to the effect that the spectra appear side by side inthe same sequence on the rows is avoided. Although such an image is notexcluded, it involves great difficulties in adjusting and interpretingwhen small individual sensors as they are found in commerciallyavailable focal plane arrays are utilized.

In the example according to FIG. 3, but 50% of the individual sensors 48are employed. If not all the individual sensors 48 of the detectormatrix 46 are interpreted, this signifies greater ease for the toppedelectronic detecting and plotting unit. Said electronics is representedin FIG. 2. The detector matrix 46 is fitted with a control unitimmediately assigned thereto. Via a represented line, this control unitfeeds the signals to a plotting station 50 which in turn is connected toa display and storage unit 52 via a line.

The already mentioned FIG. 4 shows a front view of the end of the exitof the optical fiber beam. The single exit areas 34 of each and everyoptical waveguide 28 may be recognized. The exit areas are orientedalong a straight line and are tightly packed side by side without anyspacing. The individual exit areas 34 are located in one plane. In theembodiment shown, the end regions of the individual optical waveguides28 are combined and kept together by means of extrusion.

FIG. 5, which has also been mentioned already, shows how a fiber 54coming from the left and integral with the entrance area 30 and having adiameter of approximately 0.5 micrometers is optically coupled to athinner fiber 56 of a diameter of approximately 0.05 micrometers whenusing the second exemplary embodiment. The structure used has V-shapedindentations that are immediately adjacent to each other. In therepresentation according to FIG. 5, the view is oriented onto an innerarea of such a V-indentation. The indentation is stepped so that, wheninserting the fibers 54, 56, they are substantially facing each other ina centrical way. A number of V-indentations of the type mentionedcorresponding to the total of up to m optical waveguides are formed inthe transmission body as it is shown in FIG. 5. In this way, all theoptical waveguides 28 of the second exemplary embodiment describedherein above can be reduced to fibers having diameters of approximately0.06 micrometers.

What is claimed is:
 1. A device for detecting properties of a web ofmaterial, said web of material having a surface and a longitudinaldirection and being conveyed in said longitudinal direction, said devicecomprising in combination: a crossbar, said crossbar extending acrossthe web of material; an infrared spectrometer, said infraredspectrometer having an input side and an output side; a holographicgrating which is arranged in the infrared spectrometer; infrareddetectors, where said infrared detectors are arranged at the output sideof the infrared spectrometer and are formed by a detector matrix havingn lines and m rows of infrared sensitive individual sensors; and aplurality of optical waveguides, each waveguide having an entrance areaand an exit area, said entrance area being located in vicinity of thesurface of the web of material, being oriented towards said surface andbeing fastened to said crossbar, said exit areas of the opticalwaveguides being connected to the input side, wherein the opticalwaveguides are arranged side by side in one line at the input side ofthe spectrometer, infrared spectra inputted into the entrance areas ofthe individual optical waveguides appearing in rows side by side at theoutput side of the spectrometer, and the spectra of up to m opticalwaveguides are distributed and detected in up to n spectral areas. 2.The device according to claim 1, wherein the grating has a cylindricalconvex shape.
 3. The device according to claim 1, wherein thespectrometer has an image ratio between the input side of thespectrometer and the output side of the spectrometer, and wherein theimage ratio ranges from 0.5 to 1 to 1 to 0.5.
 4. The device according toclaim 3, wherein the image ratio amounts to approximately 1 to
 1. 5. Thedevice according to claim 1, wherein the detector matrix is providedwith more than 100 rows and with more than 100 lines of infraredsensitive individual sensors.
 6. The device according to claim 1,wherein there is always at least one unused row of individual sensorslocated between two neighboring spectra on the detector matrix.
 7. Thedevice according to claim 1, wherein mirror optics is provided thatimages the input side on the output side.
 8. The device according toclaim 1, wherein the distance of the entrance areas from the web ofmaterial is smaller than 10 mm.
 9. The device according to claim 1,wherein the optical waveguides are arranged in a row side by side atregular intervals on a crossbar, and wherein a neighboring waveguide ofa certain optical waveguide at the input side of the spectrometer isalso a neighboring waveguide of said certain optical waveguide in thearrangement on the crossbar.
 10. The device according to claim 1,wherein the web of material is a web of paper.
 11. The device accordingto claim 1, wherein groups of several neighboring individual sensors aregroupwise electrically interconnected.
 12. The device according to claim11, wherein groups of four neighboring sensors are electricallyinterconnected.
 13. The device according to claim 1, wherein thedistance of the entrance areas from the web of material is smaller than5 mm.
 14. The device according to claim 1, wherein the distance of theentrance areas from the wet of material is smaller than 2 mm.
 15. Amethod for detecting properties of a web of material, said web ofmaterial having a surface and a longitudinal direction and beingconveyed in said longitudinal direction using a device comprising incombination: a crossbar, said crossbar extending across the web ofmaterial; an infrared spectrometer, said infrared spectrometer having aninput side and an output side; a holographic grating which is arrangedin the infrared spectrometer; infrared detectors, where said infrareddetectors are arranged at the output side of the infrared spectrometerand are formed by a detector matrix having n lines and m rows ofinfrared sensitive individual sensors; and a plurality of opticalwaveguides, each waveguide having an entrance area and an exit area,said entrance area being located in vicinity of the surface of the webof material, being oriented towards said surface and being fastened tosaid crossbar, said exit areas of the optical waveguides being connectedto the input side, wherein the optical waveguides are arranged side byside in one line at the input side of the spectrometer, infrared spectrainputted into the entrance areas of the individual optical waveguidesappearing in rows side by side at the output side of the spectrometer,and the spectra of up to m optical waveguides are distributed anddetected in up to n spectral areas, wherein the detector matrix isinterrogated from time to time and at each interrogation only a maximumof 80% of the n×m individual sensors are interrogated.
 16. Methodaccording to claim 15, wherein at each interrogation a maximum of 60% ofthe n×m individual sensors are interrogated.